Author's personal copy
12.5 Riparian Vegetation and the Fluvial Environment: A Biogeographic
Perspective
J Bendix, Syracuse University, Syracuse, NY, USA
JC Stella, State University of New York College of Environmental Science and Forestry, Syracuse, NY, USA
r 2013 Elsevier Inc. All rights reserved.
12.5.1
12.5.2
12.5.3
12.5.4
12.5.4.1
12.5.4.2
12.5.4.3
12.5.4.4
12.5.4.5
12.5.4.6
12.5.5
12.5.6
12.5.7
12.5.7.1
12.5.7.2
12.5.7.3
12.5.7.4
12.5.7.5
12.5.8
References
Introduction
Early History: Pattern and Process in Riparian Zones
Influence of Hydrogeomorphology on Vegetation: Evolution from Descriptive to Quantitative Studies
Specific Mechanisms of Hydrogeomorphic Impact
Flood Energy
Sedimentation
Prolonged Inundation
Water-Table Depth and Dynamics
Soil Chemistry
Propagule Dispersal
Influence of Vegetation on Geomorphology
Feedbacks between Vegetation and Hydrogeomorphology
Patterns in Published Literature
The Sample and Coding
General Characteristics of the Sampled Literature
Differences among Biomes
Scale-Related Differences
Hydrogeomorphic Mechanisms in the Context of Scale and Biogeography
Patterns and Perceptions Revealed in the Literature
Abbreviations
CHILD
Channel–hillslope integrated landscape
development
LWD
TN
TOC
53
54
55
56
56
56
57
57
58
58
59
60
62
63
64
65
66
67
68
69
Large woody debris
Total nitrogen
Total organic carbon
Abstract
In this chapter, we review the historical arc of research on biogeomorphic interactions between fluvial geomorphology and
riparian vegetation. We then report on an examination of the past 20 years of published research on this topic. Having
classified studies according to the key relationships they have identified, we map those relationships to seek spatial patterns
that emerge in terms of either physiographic environment or actual geographic location. We also consider the varied
patterns of causal interactions that emerge at different spatial scales.
12.5.1
Introduction
The recognition of riparian zones as biogeomorphic units in
the landscape is critical both for theoretical understanding and
for conservation of these high-value habitats. Since the middle
of the twentieth century, study of the systematic patterns of
vegetation communities in riparian zones has led to a greater
Bendix, J., Stella, J.C., 2013. Riparian vegetation and the fluvial
environment: a biogeographic perspective. In: Shroder, J. (Editor in Chief),
Butler, D.R., Hupp, C.R. (Eds.), Treatise on Geomorphology. Academic
Press, San Diego, CA, vol. 12, Ecogeomorphology, pp. 53–74.
Treatise on Geomorphology, Volume 12
understanding of the linked physical and biotic processes that
sustain them (Osterkamp and Hupp, 1984; Gregory et al.,
1991; Naiman and Decamps, 1997; Naiman et al., 2005). For
much of this time, emphasis was placed on the importance of
understanding hydrogeomorphic influences on riparian vegetation, because fluvial forces set the physical template for
ecological communities. Recently, there has been greater study
of the reciprocal influence of biological organisms and processes on geomorphic processes, leading to a biogeomorphic
understanding of fluvial/riparian ecosystems (Corenblit et al.,
2007). This view recognizes the substantial influence
that vegetation pattern and structure can have in altering the
http://dx.doi.org/10.1016/B978-0-12-374739-6.00322-5
53
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
distribution of physical forces of flow and sediment regimes,
which ultimately create feedbacks to biological communities
and ecosystems (Bendix and Hupp, 2000).
Early work relating riparian ecology to fluvial processes was
primarily descriptive in nature, with a common thread being
the classification of riparian vegetation communities and their
association with particular fluvial landforms, or even simply
particular vertical locations relative to stream channels. These
studies were generally place- and taxa-specific. The development of geomorphology as a more quantitative science since
the middle of the last century (e.g., Leopold and Maddock,
1953; Leopold et al., 1964) laid the basis for more generality
and a common biogeomorphic process approach to the study
of rivers and riparian zones (Osterkamp and Hupp, 2010).
Examples of this work include the hydraulic geometry approach (Leopold and Maddock, 1953), process-based classification of river environments (Montgomery and Buffington,
1997), and quantifications of interactions between sediment
transport capacity and supply across a wide range of spatial
and temporal scales (e.g., Howard et al., 1994).
However, much of the early work was conducted on North
American streams, with particular ecological communities and
physical regimes influencing a great deal of the development
in early theories and empirical examples. The histories of both
geomorphology and ecology include reminders of the danger
of generalizing theories across dissimilar environments. Critiques of the roles of William Morris Davis in the former
discipline and of Frederick C Clements in the latter have noted
that theoretical models developed in distinctive geographic
settings did not translate appropriately when imposed elsewhere. In a recent example, studies by Walter and Merritts
(2008) in eastern North America suggest that the geomorphic
context in the region where much of the early hydraulic
geometry research (e.g., Leopold and Maddock, 1953) was
conducted may have been in disequilibrium due to historical
human influences (i.e., mill dams). These well-known examples serve as reminders that processes and relationships
vary spatially. In particular, the very different environmental
conditions and vegetation structure occurring in different
biomes suggest that plant adaptations to hydrogeomorphic
constraints, and the subsequent influence of vegetation on
fluvial processes, may vary widely from one setting to another.
Another spatial complication arises when we consider the
scale at which biogeomorphic relationships are observed. A
variety of studies have indicated that the causal relationships
observed between riparian vegetation and hydrogeomorphic
processes may differ depending upon the scale of observation
(Baker, 1989; Dixon et al., 2002; Petty and Douglas, 2010). In
some, but not all, instances, the relationships at smaller scales
may be hierarchically constrained by those operating at larger
scales (Bendix, 1994a). Although there have, by now, been
several case studies in varied environments of the impact of
scale on riparian biogeomorphic environments, we lack a
systematic review of the empirical findings regarding scale
impacts – both from studies that explicitly explored scale and
from those that did not but contain relevant information.
These issues raise the questions of how geography and
scale influence our understanding of riparian zones and
their linked physical/biological processes. For example, how
do the geographic setting and scale of observation affect our
conclusions about drivers, responses, and landscape patterns
in riparian vegetation communities? In which biomes and at
what scales is the direction of influence primarily from the
physical to the biotic, the reverse, or one of strongly linked
feedbacks? How does the biome or geographic setting influence the relative strength of ecosystem drivers, including disturbance regimes (flooding, scour, and sediment deposition),
abiotic stress (e.g., seasonal drought), soil chemistry, and/or
limitations on life-history processes such as propagule
dispersal?
We believe that a biogeomorphic approach is necessary for
a legitimate understanding of either fluvial processes or the
ecology of the riparian zone. The large volume of relevant
empirical research since several landmark papers in the 1980s
and early 1990s (e.g., Osterkamp and Hupp, 1984; Hupp and
Osterkamp, 1985; Gregory et al., 1991) suggests the need to go
beyond a catalog-style review to a more analytical approach to
understanding the complex relationships between the ecological and geomorphic elements of the riparian environment.
We therefore seek to examine the literature on the interactions
between riparian plant communities and fluvial landforms/
processes, within a bipartite, spatially oriented framework that
emphasizes how biogeomorphic interactions between riparian
vegetation and fluvial geomorphology vary with both geography and the scale of observation.
In this chapter, we begin with a condensed review of the
historical arc of research on the varied fluvial impacts on
vegetation, the means whereby vegetation influences fluvial
geomorphology, and the feedbacks between the two. The bulk
of that research has been on the specific mechanisms of
hydrogeomorphic influence on riparian plant communities;
therefore, these mechanisms receive particular emphasis in
our review. We then report on an examination of the past 20
years of published research on this topic. Having classified
studies according to the key relationships they have identified,
we map both the causal directions and the mechanisms to
seek spatial patterns that emerge in terms of either physiographic environment or actual geographic location. We also
consider the varied patterns of causal interactions that may
emerge at different spatial scales.
12.5.2
Early History: Pattern and Process in
Riparian Zones
The current understanding of the physical/biological linkages in
river ecosystems is inherited from a long line of conceptual and
empirical work in stream ecology and river science. The issues
of biogeography and scale in studies of riparian ecology date to
early work relating geomorphic landforms to vegetation communities. For almost a century, scholars have acknowledged
and described associations between geomorphic landforms and
the distribution of vegetation communities in river and floodplain systems. More recently, researchers have investigated the
specific processes that drive these vegetation associations with
landform. Early studies drew on the disciplinary traditions of
geography, geomorphology, and ecology, establishing strands
that have persisted through time. Beginning in the 1930s, a
primary focus of this work was the description and classification of riparian vegetation communities, but, even in these
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
early efforts, there were attempts to systematically relate vegetation communities to fluvial landforms and processes. The
distinct zones of vegetation described by Hefley (1937) on the
low terraces of the Canadian River in Oklahoma were subsequently attributed by Ware and Penfound (1949) to a combination of flood damage and drought.
Such studies were the precursors of later work that would
use more rigorous statistical tools to document similar relationships (Hupp and Osterkamp, 1985; Harris, 1987; Shin
and Nakamura, 2005). Our understanding of process and
quantitative tools for testing this have evolved over time to the
point that some researchers have developed and tested predictive models of vegetation response to hydrogeomorphic
factors such as flood inundation (Auble et al., 1994; Dixon
and Turner, 2006), flood energy (Bendix, 1999; Sandercock
and Hooke, 2010; Stromberg et al., 2010), ice scour (Auble and
Scott, 1998), channel meandering, and floodplain sedimentation (Harper et al., 2011). Some of these models are phenomenistic (e.g., Auble et al., 1994; Shafroth et al., 1998),
whereas others are mechanistic in nature (Lytle and Merritt,
2004; Stella, 2005; Dixon and Turner, 2006; Harper et al.,
2011).
12.5.3
Influence of Hydrogeomorphology on
Vegetation: Evolution from Descriptive to
Quantitative Studies
Hydrogeomorphic processes generally influence riparian
vegetation through their contribution to the physical disturbance regime, which affects plant demography (e.g., dispersal to safe sites and flood mortality), and through their
impacts on the resource environment for plant growth. These
drivers interact with plant life-history processes, traits, and
physiological tolerances to influence population demographics and community dynamics in riparian communities.
As early as the 1930s, when plant zonation patterns on rivers
were first documented, researchers had recognized that
flooding regimes influence the composition, distribution, and
structure of riparian vegetation. Illichevsky (1933) documented distinct cross-sectional compositional belts in the
floodplain of the Dnieper River, noting differences in plant
assemblages between the inundated and dry zones. Examples
of similar work in the years that followed include Shelford’s
(1954) documentation of biotic communities in the lower
Mississippi valley floodplain and their age and elevation
relative to the mean low water level, and Fanshawe’s (1954)
use of topographic position on bars and banks as a criterion
for distinguishing plant communities on river fringes in British Guiana.
Studies during this early period generally described patterns more than the processes that influenced them, although
there are some notable exceptions (Hefley, 1937; Ware and
Penfound, 1949), in particular, investigations relating vegetation to flood frequency and duration, and linking flood
disturbance to successional zonation (Wistendahl, 1958;
Lindsey et al., 1961). Sigafoos (1961) took issue with successional interpretations, arguing that zonation instead reflected
tolerance to flooding (see Hupp (1988) for a useful discussion of where and why successional zonation might
55
occur). Sigafoos identified distinctive vegetation bands
along the Potomac River related to inundation frequency;
these observations set the stage for his groundbreaking work
reconstructing past floods from dendrogeomorphological
records (Sigafoos, 1964). Although much of the research in
this period was in the middle and eastern parts of North
America, valuable research emerged in the West as well. Everitt
(1968) related cottonwood regeneration on Utah’s Fremont
River to flood dynamics. Teversham and Slaymaker (1976)
studied riparian vegetation composition in Lillooet River valley, British Columbia, documenting species groupings that
were related to both sediment size and flood frequency. These
examples, by no means exhaustive of the studies during this
period, give an indication of the range of contemporary
perspectives.
Beginning in the 1980s, interest in riparian ecology
broadened greatly, leading to classification systems of vegetation communities with landforms and quantification of the
physical processes that sustained them. Osterkamp and Hupp
(1984) (also see Hupp and Osterkamp, 1985) investigated
these relationships along northern Virginia streams and
documented assemblages of woody plants on geomorphic
surfaces that are distinguished by inundation frequency and
duration. Similar studies using categorical classifications of
landforms that are associated with different flood inundation
frequencies include examples from Northern California
(Harris, 1987), Central Oregon (Kovalchik and Chitwood,
1990), and Japan (Shin and Nakamura, 2005). In this approach, vegetation species presence and/or cover was recorded
and associated with elevation above the active channel, and by
association with flood frequency and duration. Often, some
type of multivariate analysis was used to classify species into
groups that sort along hydrologic gradients (Harris, 1987;
Auble et al., 1994; Bendix, 1994b; Rodrı́guez-González et al.,
2010).
Beginning in the 1990s, researchers began to develop
quantitative models linking vegetation response to mechanistic, hydrological, and hydraulic drivers. In an innovative
approach combining standard numerical modeling in plant
ecology and river engineering, Auble et al. (1994) conducted a
plant community analysis of the Gunnison River (CO), using
cluster analysis in conjunction with hydraulic modeling to
quantify the flood duration of distinct plant communities and
model vegetation change under proposed regulated flow regimes. In California’s Transverse Ranges, Bendix (1999) used
hydraulic modeling, ordination, and regression to relate plant
community composition to computed values of unit stream
power, as opposed to earlier studies that relied on assumptions of relative flood energy inferred from landform position.
His research demonstrated that variation in hydraulic parameters imposes spatial variation in the ecological impact of
floods within watersheds (Bendix, 1997, 1998). In another
application of hydraulic modeling, Dixon et al. (2002) integrated calculations of energy slope from a one-dimensional
hydraulic model with quantitative landscape analysis to investigate the influence of physical characteristics at the local
and landscape scales on the distribution of pioneer tree
seedlings along the Wisconsin River. This was an extension of
the approach WC Johnson used to infer maximum streamflow
thresholds that cause seedling mortality from flood removal
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
and ice scour on both the Missouri River (Johnson et al., 1976;
Johnson, 1992) and the Platte River, Nebraska (Johnson,
1994, 1998).
With the increasing emphasis on sophisticated quantitative
analyses, the mechanisms of plant recruitment and mortality
were studied in more detail, and species-level investigations assumed greater prominence to complement studies conducted at
the whole-community level (Shafroth et al., 1998; Stella, 2005;
Dixon and Turner, 2006; Shafroth et al., 2010). Riparian species
differ in key life-history traits that are linked to recruitment and
survival in dynamic fluvial environments, including inundation
duration (Huffman, 1980; Toner and Keddy, 1997; Robertson
et al., 2001; Kozlowski, 2002; van Eck et al., 2004), seed dispersal and characteristics of fecundity, release timing, seed longevity and buoyancy (Kubitzki and Ziburski, 1994; Danvind and
Nilsson, 1997; Lopez, 2001; Nilsson et al., 2002; Stella et al.,
2006; Gurnell et al., 2008), and those related to survival of scour
and burial (Bornette et al., 2008), particularly hydraulic resistance and stem flexibility, rooting depth, and resprouting ability
(Brewer et al., 1998; Levine and Stromberg, 2001; Karrenberg
et al., 2002; Lytle and Poff, 2004; Renofalt and Nilsson, 2008;
Rodrı́guez-González et al., 2010). Differences in reproductive
and survival traits have been used successfully in several mechanistic, numerical approaches to predict relative abundance of
species within riparian communities (Shafroth et al., 1998;
Stella, 2005; Dixon and Turner, 2006; Bornette et al., 2008).
12.5.4
Specific Mechanisms of Hydrogeomorphic
Impact
Hydrogeomorphic processes interact with riparian vegetation
through life-history characteristics, plant traits, and physiological tolerances (Rood et al., 2003). There are many mechanisms that influence vegetation composition, distribution,
and density; however, for ease of analysis we have grouped
them into six main categories, all of which are consequences
to some degree of periodic floods and the dynamic hydrology
characteristic of near-channel environments:
1. flood energy, which exerts a limiting effect on vegetation
distribution due to scour and stem breakage;
2. sedimentation, which can exert influence through creation
of new habitat for plant colonization and reduction of
competition, burial of existing vegetation, and/or sediment
texture effects on water availability;
3. prolonged inundation, which generally reduces physiological function and survival;
4. water-table depth and dynamics, which regulate soil
moisture;
5. soil chemistry influences, including mineral nutrition, salinity, and pollutants; and
6. fluvial controls on propagule dispersal.
12.5.4.1
Flood Energy
The hydraulic energy associated with flooding in dynamic
river systems causes damage or mortality of plants through
root zone scour and stem breakage (Hughes, 1997; Polzin and
Rood, 2006; Perucca et al., 2007). Vegetation is particularly
vulnerable when plants are small relative to the magnitude of
flood energy, such as during seedling establishment. This is
also the case for plant removal by scour, which will generally
occur when scour depth approaches the depth of coarse roots.
Root or stem breakage not only is common for herbaceous
vegetation during high flow but also can occur for woody
plants in large floods (Chambers et al., 1991; Groeneveld and
French, 1995; Gurnell et al., 2002). The energy required to
remove or break plants varies with their size and flexibility,
their root characteristics, and the nature of the substrate in
which they are rooted (Bendix, 1999).
At the vegetation patch scale, plant removal occurs when
scour depth exceeds a critical depth that is related to the size
and structure of the vegetation roots. The patchy nature of
riparian vegetation introduces complexities and feedbacks to
flood energy because vegetation often reduces near-bed velocities and scour depths deep within a patch, but increases
velocities at the upstream and outside edges of vegetation
patches through flow contraction. This can create wider and
deeper scour patterns than those produced around individual
stems and increased susceptibility to flood mortality for exposed plants (Lightbody et al., 2008; Yager and Schmeeckle,
2007).
The impacts of flood energy on the species composition of
riparian plant communities may be seen both through differential survival of the potential damage described above and
through colonization by pioneers replacing the species that do
not survive (Bendix and Hupp, 2000). More subtly, even
floods that do not cause mortality may clear leaf litter, providing an advantage for species that require bare mineral soil
for colonization (Yanosky, 1982).
12.5.4.2
Sedimentation
Sedimentation can act on plants in either positive or negative
ways, and includes effects of landform construction (Cooper
et al., 2003; Latterell et al., 2006), mortality due to burial
(Hack and Goodlett, 1960), and sediment texture controls on
water availability (McBride and Strahan, 1984a). At the
landscape scale, the sediment regime has a controlling influence on plant establishment, community composition, biomass, and vegetation structure (Everitt, 1968; Hickin, 1984;
Bechtold and Naiman, 2006; Naiman et al., 2010). Channel
migration processes drive formation and development of
geomorphic surfaces to control the establishment and spatial
extent of pioneer communities (McKenney et al., 1995; Lytle
and Merritt, 2004; Harper et al., 2011). Increasingly, studies
have focused on description and quantification of the relative
importance of different establishment pathways for riparian
forest stands on alluvial surfaces, including point bars, abandoned channels, and mid-channel bars (Baker and Walford,
1995; Cooper et al., 2003, 2006; Latterell et al., 2006; Van Pelt
et al., 2006; Stella et al., 2011). One of the primary influences
of new landforms on colonizing vegetation is through sediment texture effects on moisture-holding capacity, which in
turn controls plant survival and growth (Mahoney and Rood,
1992; Hughes et al., 2000; Hupp and Rinaldi, 2007).
At smaller spatial scales, sedimentation affects plants
through mortality by burial. The severity of this effect depends
on the size of the plant relative to sediment deposition rate,
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
which is related to the transport capacity of individual flood
events, basin-scale sediment supply (Buffington and Montgomery, 1999), meso-scale influences such as bar topography
(Dietrich and Whiting, 1989; Francis and Gurnell, 2006), and
local roughness fields from live vegetation or woody debris
(Gurnell and Petts, 2006; Yager and Schmeeckle 2007).
Many plant species can survive burial and resprout from
epicormic buds (Bond and Midgley, 2001). However, the severity of impact (i.e., plant mortality rate) depends on the
depth of sediment deposited during an event, as well as the
plant size, season (dormant vs. active), and taxon-specific
physiology. Ewing (1996) found decreased growth and
physiological function by alder and sedge plants after partial
sediment burial. Levine and Stromberg (2001) found increased resistance by cottonwood over tamarisk to sediment
burial, and other work along the Bill Williams River, AZ, has
documented substantially greater mortality of tamarisk compared to willow seedlings in response to two dam-controlled
flood releases (Shafroth et al., 2010). Mortality occurred by
both scour and burial and was likely nonproportional among
species because of the substantially greater first-year height
and diameter growth of willow relative to tamarisk (Sher et al.,
2002; Shafroth et al., 2010).
12.5.4.3
Prolonged Inundation
Although temporary flooding generally increases the supply of
water and nutrients to terrestrial plants (Burke et al., 1999;
Clawson et al., 2001; Kozlowski and Pallardy, 2002), longterm soil saturation induces soil anoxia, which limits nutrient
availability and gas exchange for plants (Mitsch and Gosselink, 2007), and soil toxicity due to reducing conditions. These
negative effects are mitigated to some degree by plants’
adaptive responses to prolonged flooding, including tissues to
facilitate oxygen exchange such as aerenchyma and lenticel
development (Blom and Voesenek, 1996; Crawford, 1996;
Rood et al., 2003; Walls et al., 2005). In riparian and other
ecosystems where stressful abiotic conditions may limit plants’
size, life span, and/or recruitment opportunities, increased
sprouting has been associated with waterlogged soils
(Rodrı́guez-González et al., 2010), and may be an important
strategy in particular for species that do not maintain a seed
bank (Bond and Midgley, 2001; Nzunda et al., 2007).
Much of the research on riparian plants’ physiological responses to inundation is based on laboratory experiments
(e.g., Pereira and Kozlowski, 1977; Conner et al., 1997; Li
et al., 2005; Day et al., 2006), with a somewhat lesser emphasis on field studies (Megonigal et al., 1997; Eschenbach
and Kappen, 1999; Tardif and Bergeron, 1999). In the field,
variation in vegetation across slight topographic gradients
has often been attributed to differential tolerance of inundation (Bell, 1974; Nixon et al., 1977; Robertson et al., 1978).
But in observational field studies, inundation duration is often
correlated with other mechanisms such as maximum flood
energy, sediment texture, nutrient availability, and redox potential (Rodrı́guez-González et al., 2010). Therefore, teasing
out specific contributions of inundation alone is difficult, especially as these effects are nonlinear and in some cases contradictory. For example, various studies on the influence of
inundation on tree growth have found negative effects (e.g.,
57
Mitsch et al., 1991; Megonigal et al., 1997; Rodrı́guez-González et al., 2010), whereas others have found them to be
positive (Burke et al., 1999; Clawson et al., 2001; Hanson
et al., 2001). Some of these contradictions may reflect geographic variation, as environments typified by prolonged
hydroperiod are likely to support species that show a positive
response to inundation (Hupp, 2000).
12.5.4.4
Water-Table Depth and Dynamics
Although many riparian plant species are well adapted to their
dynamic river systems through traits such as abundant seed
production, wind dispersal, and fast growth (Karrenberg et al.,
2002; Lytle and Poff, 2004), there are generally life-history
tradeoffs that include demand for abundant soil moisture and
intolerance to drought. In particular, survival of seedlings in
arid and semi-arid climates is a challenge because seasonally
fluctuating water tables and severe vapor pressure deficits can
dramatically reduce water availability during the critical establishment stage (Donovan and Ehleringer, 1991; Horton
and Clark, 2001; Hughes et al., 2001; Rood et al., 2003). On
snowmelt-dominated rivers, extended flow and groundwater
declines typically occur in late spring and early summer, as the
snowpack melts (Peterson et al., 2000). Seedlings and other
shallowly rooted plants are particularly vulnerable, and seasonal water shortage can act with other stressors in the riparian zone (e.g., scour, herbivory, and competition with
herbaceous species) to limit population dynamics among riparian plants (Lytle and Merritt, 2004; Scott et al., 1996;
Stromberg et al., 1991).
In arid and semi-arid regions, the connection of stream to
riparian water table plays a critical role in resource supply and
plant survival (Zimmerman, 1969; Rood et al., 2003), and can
be the limiting factor for seedling survival and the population
structure of pioneer plants (Lytle and Merritt, 2004). Longterm alterations in flow magnitude, timing, and recession rate
have exacerbated the effects of seasonal water limitation in
these systems and threaten to further reduce the extent of riparian woodlands (Fenner et al., 1985; Rood and Mahoney,
1990; Rood et al., 1999; Stella, 2005; Braatne et al., 2007); nor
is the importance of water-table proximity limited to dry environments. On the floodplain of New Jersey’s Raritan River,
Frye and Quinn (1979) interpreted species distributions as a
function of soil texture and depth to water table. In this setting, they considered a shallow water table to be a limiting
factor, arguing that it excluded deep-rooted species. Numerous
studies in lowland riparian environments highlight these
species-specific differences in soil saturation and toleration of
root anoxia (Niiyama, 1990; Conner et al., 1997; Rodrı́guezGonzález et al., 2010).
Despite their vulnerability to drought, riparian plants
demonstrate some mitigating morphological and physiological
traits. Annual species avoid drought by completing their life
cycle during wet seasons. For perennial plants, rapid root extension and small shoot:root biomass ratios are adaptations
that potentially reduce stresses related to seasonally variable
water tables (Amlin and Rood, 2002; Horton and Clark, 2001;
Hughes, et al., 1997; Kranjcec et al., 1998; Segelquist et al.,
1993). Other morphological responses to water stress include
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
reduction in leaf size (Stella and Battles, 2010), specific leaf area
(Busch and Smith, 1995), crown dieback (Scott et al., 1999),
branch abscission (Rood et al., 2000), and reduced diameter
growth (Stromberg and Patten, 1996). Studies of physiological
function in adults indicate that these species are generally intolerant of drought and have low xylem cavitation thresholds
(Amlin and Rood, 2003; Cooper et al., 2003; Leffler et al.,
2000; Tyree et al., 1994). Higher water-use efficiency as indicated by higher d13C values is a common response of
water-stressed plants (Smedley et al., 1991) and has been observed for riparian seedlings under experimental drought
(Zhang et al., 2004) and adult natural populations between
wet and dry years (Leffler and Evans, 1999). Watertable manipulations in controlled mesocosms have been used
to simulate dynamic riverine environments in order to
study the effects of seasonal moisture stress on plant survival
and growth (e.g., Cordes et al., 1997; Horton and Clark,
2001; Mahoney and Rood, 1992; Segelquist et al., 1993; Stella
et al., 2010). In one study, Stella and Battles (2010) found
that seedlings of related species grown under identical conditions of water stress displayed different adaptations, with
cottonwood minimizing specific leaf area to a greater degree
than willow, which was more effective at reducing stomatal
conductance and leaf size.
12.5.4.5
Soil Chemistry
Nonhydrologic factors also contribute to riparian plant distributions, including resource competition, lack of soil fertility, and salinity (Shafroth et al., 1995; Auble and Scott,
1998; Sher et al., 2000). Detailed work on establishment
processes on fluvial landforms, including point bars, banks,
and abandoned channels, has highlighted the importance of
the soil environment, particularly nutrients and organic matter (Van Cleve et al., 1996; Bechtold and Naiman, 2006,
2009). In a study by Kalliola et al. (1991) on riparian forest
colonization in western Amazonia, distinct vegetation communities colonized four common fluvial landforms: channel
bars, swales, abandoned channels, and riverbanks. These
newly deposited fluvial sediments are poor in organic carbon
and nitrogen, are affected by seasonal fluctuations in the rivers, and support vegetation patches that tend to be narrow,
curved, or linear patches. Local site and colonizing vegetation
characteristics vary considerably between the different river
types (e.g., meandering or braided, rich or poor in suspended
sediment).
Bechtold and Naiman (2006) studied nutrient storage and
mineralization along a toposequence on the Phugwane River
in South Africa, and found that total organic carbon (TOC),
total nitrogen (TN), and potential N mineralization were
strongly linked to particle size distributions, with TOC and TN
positively correlated with silt and clay concentration. After
accounting for the effect of particle size, landform differences
were not predictive of nutrient dynamics; therefore, they
concluded that a collinear gradient of soil texture across alluvial landforms constitutes the primary nutrient influence on
riparian soil and plant community succession.
Soil salinity is commonly cited in dryland riparian systems
as a driving factor determining plant distributions, productivity, and species’ relative competitiveness (Di Tomaso, 1998;
Gasith and Resh, 1999; Cramer and Hobbs, 2002). Glenn
et al. (1998) found higher salinity tolerance by saltcedar
(Tamarix spp.), a non-native shrub in the US Southwest,
compared to native riparian species, and concluded that this
abiotic factor was important in facilitating invasion by the
saltcedar throughout the region’s arid and saline riparian soils.
Sher et al. (2002), however, found that salinity and other soil
abiotic variables were much less important in predicting the
mortality rate of first-year tamarisk seedlings than the density
of native competitors (cottonwood and willow), which survived across a range of substrates.
12.5.4.6
Propagule Dispersal
Hydrogeomorphic influences on plant dispersal constitute
another important mechanism affecting riparian vegetation, in
part, because of co-evolution of propagule traits with disturbance regimes (Lytle and Poff, 2004). Unlike in most terrestrial environments, many plants in riparian ecosystems
have little dependence on a persistent seedbank, particularly
where physical disturbance from periodic flooding precludes
seed storage (Pettit and Froend, 2001). Lacking a mechanism
for multiyear seed storage, riparian plant recruitment typically
results from dispersal of short-lived seed directly from parent
plants (Young and Clements, 2003a, 2003b; Stella et al.,
2006), or else transport of plant fragments as propagules
(Gurnell et al., 2008). For seeds, dispersal is primarily by wind
(anemochory) and water (hydrochory). The latter frequently
depends upon synchronization of reproduction and seed
dispersal with hydrological regimes that create favorable substrate and moisture conditions for seed dispersal (Hupp,
1992), and rapid germination and recruitment (Siegel and
Brock, 1990; Scott et al., 1996; Mahoney and Rood, 1998;
Stella et al., 2006). Vegetative reproduction via flood dispersal
of plant fragments has also been observed for a range of taxa
(Gurnell et al., 2008), from willows (Douhovnikoff et al.,
2005) to columnar cacti (Parker and Hamrick, 1992).
Hydrochory may be most important during overbank flows
when propagules are dispersed across floodplains (Schneider
and Sharitz, 1988; Gurnell et al., 2006, 2008). Wind dispersal
also occurs preferentially along longitudinal stream corridors
because local channel topography and the morphology of riparian canopies often serve to guide prevailing winds (Devitt
et al., 1998).
As a result of these patterns, hydrogeomorphic forces are
highly effective at accomplishing dispersal in both longitudinal and lateral dimensions along stream channels (Merritt
and Wohl, 2002). Propagule deposition is not uniform,
however, because of variation in propagule source density
(Clark et al., 1999), substrate exposure (Johnson, 1994), and
local variations in water velocity, hydraulic roughness and
form drag due to channel morphology, existing vegetation
structure and density, bed and bank substrate, and large
woody debris (Johansson et al., 1996; Merritt and Wohl, 2002;
Pettit and Naiman, 2006). Field studies (e.g., Gurnell et al.,
2008), flume experiments (e.g., Merritt and Wohl, 2002), and
models (Levine, 2003) have attempted to quantify and predict
patterns of riparian propagule dispersal, and the effects of
dams on riparian plant distributions through interruption of
dispersal patterns have been documented (Nilsson and
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Jansson, 1995; Jansson et al., 2000). Although these studies
have provided details from a variety of settings, the complex
nature of fluvial hydrodynamics, heterogeneous riparian environments, and variation in seed sources and morphologies
currently preclude a comprehensive, predictive understanding
of dispersal.
12.5.5
Influence of Vegetation on Geomorphology
In contrast to the study of physical factors on vegetation,
which have been conducted primarily by ecologists and biogeographers, research on the influence of vegetation on fluvial
geomorphic processes and landforms has been conducted
primarily in the fields of geomorphology (typically from
physical geography and earth sciences) and civil and environmental engineering (Darby, 1999; Corenblit et al., 2007;
Sandercock et al., 2007). A long history of research exists on
vegetation influence over the hydrological cycle (e.g., Tabacchi
et al., 2000), on ecophysiological fluxes (e.g., Roberts, 2000),
and on water quality (e.g., Dosskey et al., 2010). However,
until relatively recently, quantitative research on vegetation
controls over hydrogeomorphic processes was generally descriptive (e.g., Nanson and Beach, 1977), or fairly limited in
scope and in the complexity of processes observed (Murray
et al., 2008).
During flooding, woody plant stems and canopies add
drag to a fluvial system, influencing both hydraulics and
sediment dynamics (Nepf, 1999; Lightbody and Nepf, 2006).
In addition, vegetation roots increase erosion resistance of
banks, and trapping sediment adds cohesion to the substrate
(Knighton, 1984). Corridor-wide fluvial characteristics and
processes affected by vegetation include velocity, flood stage,
flow resistance, and sediment transport (Murray et al., 2008).
Local geomorphic influences include bank erosion resistance,
bar sedimentation, formation of logjams, and floodplain
sedimentation rates (Hickin, 1984).
Efforts to predict the contribution of vegetation to velocity
date back to the nineteenth century, when Manning integrated
empirical research by Darcy, Weisbach, St. Venant, Ganguillet,
Kutter, and others to develop his equation for flow resistance
in vegetated channels (Manning, 1891). The roughness coefficient n in the Manning equation represents the collective
drag exerted on the flow by surface roughness (including
vegetation) and channel sinuosity. Subsequent research has
disaggregated this term into the sum of various environmental
components, including vegetation (e.g., Cowan, 1956). The
widespread use of Manning’s equation across a range of
hydrological, geomorphological, and engineering applications
makes the determination of Manning’s n profoundly important. Of the components that contribute to n, vegetation is
among the most variable and, hence, critical (Arcement and
Schneider, 1989). Furthermore, the changes in resistance that
accompany plant growth (or loss) mean that the roughness
coefficient may be quite variable through time (Chow, 1959).
Darby (1999) modified existing hydraulic models to predict stage-discharge curves for channels with nonuniform
cross sections, sand and gravel-bed materials, and flexible or
nonflexible riparian vegetation. Because roughness depends
on vegetation stature, density, and flexibility, empirical
59
estimates of n vary based on vegetation community type and
time of year, and it is important to note that this method of
quantifying vegetation effects is not mechanistic, nor does it
take into account interactions between vegetation drag and
hydrodynamic forces (Corenblit et al., 2007).
More recently, researchers have begun to quantify the effects of vegetation on flow velocities, both as one-way processes (involving rigid stems) and interactive ones (involving
flexible vegetation). Experimental and theoretical work involving rigid, nonsubmerged stems approximate the effects of
woody plants on overbank floods, and show that the diameter,
density, and clustering of stems affect both flow velocity
and stage. Stone and Shen (2002) conducted flume experiments with cylinders to represent rigid plant stems of various
sizes and densities. They showed that flow resistance varies
with density, length, and diameter of stems, as well as flow
depth, and developed physically based formulas for resulting
flow resistance and velocity. Nepf (1999) extended the
work on rigid vegetation to emergent stems with feedbacks
to vegetative drag and flow turbulence, and developed a
model to describe the drag, turbulence, and diffusion for flow
through emergent vegetation with varying properties of stem
density.
The role of large woody debris (LWD) in the initiation and
evolution of geomorphic landforms, first described by Keller
and Swanson (1979), has received substantial attention over
the last two decades (e.g., Bilby and Ward, 1991; Fetherston
et al., 1995; Abbe and Montgomery, 1996; Gurnell et al., 2002;
Jeffries et al., 2003; Lassettre et al., 2008; Opperman et al.,
2008). Woody debris loading and transport have been correlated with in-channel sediment storage (e.g., Bilby and
Ward, 1991), pool spacing (e.g., Montgomery et al., 1995),
channel island formation (e.g., Gurnell et al., 2002), floodplain accretion (e.g., Jeffries et al., 2003), and increased
channel complexity (e.g., Pettit et al., 2006; Abbe and
Montgomery, 1996), among other features. Abbe and Montgomery (1996) documented the initiation of channel-altering
jams when key-member logs become lodged in the channel.
Over time, these jams collect more LWD and sediment, and
form stable structures over 101–102 years that control local
channel hydraulics and ultimately assist in riparian forest
development. Jeffries et al. (2003) documented that LWD on
floodplains increased the rate and the variability of overbank
flooding and deposition depths.
LWD interacts with sediment to exert strong effects on alluvial channel morphology, particularly where longitudinal
slopes are gentle enough to trap bedload behind logjams. In a
study of LWD effects on channel habitat in the Pacific
Northwest (US), Montgomery et al. (1995) found that pool
spacing depended on LWD loading and channel type, slope,
and width. Mean pool spacing in channels with gentle slopes
decreased 13-fold with increased LWD loading, whereas pool
spacing in steeper channels was not affected. From their study
of sediment processes on the Tagliamento River (Italy),
Gurnell et al. (2001) developed a conceptual model of island
development that integrates interactions between LWD and
vegetation, geomorphic features, sediment texture, and
hydrological regime. Depositional and erosional processes
resulted in different island types and floodplain developmental stages.
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The potential for LWD to resprout or for live trees to form
logjams in stream channels is another mechanism by which
vegetation affects geomorphic processes, often with longerterm effects than for dead wood (Opperman et al., 2008).
Following a 100-year flood on the Sabie River in South Africa,
Pettit et al. (2006) found that patterns of LWD accumulation
were dominated by characteristics of the trapping sites, especially trees that fell but remained rooted in place and
resprouted (36% of piles surveyed). These resprouted piles
supported tree seedlings (28% of piles) and created heterogeneous topography in the main channel and floodplain.
Opperman and Merenlender (2007) found that living trees in
Northern California streams represented a major portion of
the functional in-stream large wood, was 28% more persistent
than LWD, and was likely more stable than dead wood of
similar size. They concluded that live wood may have a disproportionate influence on channel morphology, particularly
in riparian ecosystems lacking many large diameter trees.
Bank cohesion is another important geomorphic characteristic that is at least in part a function of vegetation attributes, including erosion resistance conferred by roots and
increased cohesion from fine sediment deposition induced by
plant hydraulic roughness (Knighton, 1984; Murray et al.,
2008). In an early example, Clifton (1989) showed that
morphology of mountain streams varied with vegetation
structure, in addition to physiography and land use. Abernathy
and Rutherford (1998) developed a classification scheme for
assessing the role of vegetation in stream bank erosion at
different points throughout a catchment. Their experimental
work focused on the enhancement of bank strength by reducing pore-water pressures and by directly reinforcing bank
material by plant roots (Abernathy and Rutherford, 2001).
Experimental work by Simon and Collison (2002) quantified
bank strength effects on constituent hydrologic and mechanical processes, including soil moisture modification and root
reinforcement, as well as the potentially destabilizing influence of vegetation weight, and tested their effects on the
probability of bank failure. Tree and grass roots exerted species-specific increases on soil strength, and mechanical effects
of plant cover increased stability by 32–71%. Martin and
Church (2000) found that the addition of roots to riverbanks
improved stability even under worst-case hydrological conditions, and was apparent over a range of bank geometries,
with the best protection offered by trees closest to the potential bank failure locations. Much of the work on root reinforcement has been based on models derived from materials
science; however, Pollen and Simon (2005) noted that the
results derived can vary widely depending on the sophistication of the models used.
At a landscape scale, increased bank cohesion due to
belowground vegetation effects is instrumental in controlling
river meander rates (Micheli et al., 2004) and frequency of
channel cutoffs (Micheli and Larsen, 2011). Hupp (1992) cited
the combined impact of stabilization by roots and sedimentation due to the flow resistance from plant stems in recovery
and meander initiation along formerly channelized streams.
Graf (1978) found that following the introduction of tamarisk
to upper reaches of the Colorado and Green Rivers, the combination of bank stabilization and overbank sedimentation led
to the development of stable islands and dramatic channel
narrowing. He suggested that within 25 years after the species’
introduction, it had driven the system into a new equilibrium
condition. Numerous subsequent studies have also shown
vegetation colonization of former active channel bars and
banks to reduce channel planform width and width/depth
ratios, as well as change channel cross-sectional geometry (e.g.,
Friedman et al., 1996a, 1996b; Johnson, 1994, 1998; Trush
et al., 2000; Liébault and Piégay, 2001). Although these developments can be induced by natural processes such as temporary increases in precipitation (e.g., Martin and Johnson, 1987)
and recovery from large floods (Liébault and Piégay, 2002),
many are the result of dams and other human modifications to
physical processes that reduce sediment supply (e.g., Kondolf
et al., 2007) or magnitude and frequency of high-flow events
(e.g., Johnson, 1998; Michalková et al., 2011).
Recent experimental work highlights the importance of
vegetation in transforming braided streams into single-thread
channels in coupled stream–floodplain systems (Tal and
Paola, 2007, 2010; Murray et al., 2008; Braudrick et al., 2009).
Gran and Paola (2001) and Tal and Paola (2007) have
shown in flume settings that floodplains supporting vegetation (alfalfa sprouts) produced more single-thread river
planforms than braided ones, and that the resulting channels
were narrower, deeper, and less mobile. Furthermore, the experimental addition of vegetation to previously braided
channels has induced self-organization of the channel network, including formation of a single thread, control of bend
and bar migration rates, and formation of channel cutoffs
(Braudrick et al., 2009; Tal and Paola, 2010).
However, the magnitude of vegetation effects is highly
context dependent. Labbe et al. (2011) found that riparian
vegetation was not a significant control on channel crosssection form on the Tualatin River, Oregon. Sandercock et al.
(2007) noted that in contrast to humid climates, semi-arid
and arid climate riparian zones are characterized by locally
patchy vegetation and channel morphology influenced by
extreme flood events. Therefore, the role of vegetation in bank
stability in these environments is expected to be lower overall
than in humid climates, and the degree of influence highly
dependent on local distribution, composition, and density
(Sandercock et al., 2007).
12.5.6
Feedbacks between Vegetation and
Hydrogeomorphology
If, as discussed above, hydrogeomorphic factors influence
vegetation, and vegetation influences hydrogeomorphic processes, then it follows that there are likely to be feedbacks
between the two. Indeed, not only are there interactions between vegetation and the hydrogeomorphic mechanisms described in Sections 12.5.4.1–12.5.4.6, but many of those
mechanisms also interact with each other. A diagram of
these interactions serves both to summarize many of the
relationships described in this chapter and to illustrate
the complexities of riparian biogeomorphic feedbacks (Figure 1). The relationships shown may be briefly described as
follows:
a. Flood energy affects riparian vegetation directly, through
mechanical damage or scour, as discussed in Section 12.5.4.1.
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Flood
energy
c
a
Sedimentation
o
b
d
Propagule
dispersal
p
g
e
Vegetation
f
n
k
Inundation
61
m
j
Soil
chemistry
h
i
Water
table
l
Figure 1 The network of potential biogeomorphic interactions and feedbacks that may link riparian vegetation and fluvial geomorphology.
Influences represented by the arrows are discussed in the text.
b. Riparian vegetation affects flood energy through hydraulic
roughness (Section 12.5.5).
c. Flood energy affects sedimentation, controlling the size
distribution of mobilized sediment, its sorting, and the
spatio-temporal patterns of deposition.
d. Sedimentation affects riparian vegetation directly, through
burial of existing plants and through creation of alluvial
surfaces for colonization (Section 12.5.4.2).
e. Sedimentation affects soil chemistry, through the composition of the alluvium deposited and particle size controls on nutrient dynamics and mineralization rate.
f. Sedimentation affects water-table depth through the
height of depositional landforms and the texture of the
deposits (i.e., capillarity).
g. Sedimentation affects inundation through the height of
depositional landforms.
h. Inundation affects riparian vegetation directly, through
anoxia (Section 12.5.4.3).
i. Inundation affects water-table depth and dynamics
through contributions to groundwater.
j. Water-table depth and dynamics affect riparian vegetation
directly, through provision of water or through limiting
the aerated rooting zone (Section 12.5.4.4).
k. Riparian vegetation affects the water table through
transpiration.
l. The water table affects soil chemistry through its impact
on redox potential.
m. Soil chemistry affects riparian vegetation directly, by
serving as the primary control of nutrient availability
(Section 12.5.4.5).
n. Vegetation affects soil chemistry through plant litter and
mineralization rate in the root zone.
o. Flood energy provides transport for propagules
(Section 12.5.4.6).
p. Dispersal of propagules directly affects vegetation, allowing its establishment at new sites.
It is important to note that some of the most important
relationships are actually indirect. For example, riparian
vegetation, through its impact on flood energy (b) has a very
strong influence on sedimentation (c). Overall, Figure 1 serves
to illustrate the centrality of feedbacks in the riparian zone.
Some or all of these feedbacks have been noted in earlier
reviews, including those by Parker and Bendix (1996), Bendix
and Hupp (2000), Hupp and Bornette (2003), Steiger et al.
(2005), and Corenblit et al. (2007). A limited number of
empirical and review studies have specifically addressed the
issue of feedbacks. Although these feedbacks were addressed
in a few early studies (see below), most such work has been
published since the mid-1990’s. Murray et al. (2008) referred
to the feedbacks between organisms, physical processes, and
morphology as ‘biomorphodynamics.’ They attributed the increased attention to such interactions in a variety of environments (not limited to fluvial/riparian settings) to the increased
popularity of interdisciplinary research, the challenges of
realistically analyzing responses to environmental change, and
to the advances in modeling capabilities at a variety of scales.
In his argument for application of complexity theory to
biogeomorphology, Stallins (2006) argued for a focus on feedbacks, stating a need to ‘‘move beyond listbound descriptions of
unidirectional interactions of geomorphic and ecological components.’’ He emphasized the importance of recursivity, whereby
the interactions between floods and vegetation constrain future
development of the system, so that it becomes self-organizing.
This notion is inherent in the view of Corenblit et al. (2009)
who argued that the self-organizing properties of riparian systems reflect both contemporary feedbacks that allow for niche
construction by plants and longer-term natural selection as
plants evolve for specific roles within the system.
Empirical studies of feedbacks have focused primarily on
pathways whereby riparian vegetation promotes sedimentation
through increased hydraulic roughness by live, rooted stems, or
through the role of LWD; in turn, the increased sedimentation
affects the vegetation able to grow at a site. In some instances,
these feedbacks become part of the mechanism whereby succession occurs.
In an early example of feedbacks centered on roughness,
Nanson and Beach (1977) related overbank sedimentation
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and forest succession on the Beatton River floodplain in
British Columbia. They found that dense balsam poplar colonizing new alluvial surfaces promoted increased sedimentation, for which the poplar had a high tolerance. Eventually,
however, aggradation reduced sedimentation sufficiently for
shade-tolerant white spruce to replace the intolerant poplar.
As the spruce matured, sedimentation declined even further
because the lower-stem density of the mature spruce forest
reduced hydraulic roughness. On point bars of Dry Creek in
California, McBride and Strahan (1984b) noted that colonization of gravel substrate by willows and cottonwoods resulted in deposition of finer sediments. The finer substrate
allowed for subsequent establishment of alder, which eventually formed a canopy that ultimately inhibited the shadeintolerant willows and cottonwoods. Along rivers in the Ozarks, McKenney et al. (1995) described models whereby
channel migration was driven by the age of the vegetation on
gravel bars. Young, dense stands or patches of riparian vegetation created high roughness and favored deposition and
stabilization of the surface. As the vegetation aged and selfthinning led to reduced stem densities, roughness decreased,
and the surfaces actually became less geomorphically stable.
Pettit and Naiman (2006) provided an instance in which
LWD is central to riparian feedback. A large flood on the Sabie
River (geomorphic process) not only devastated the existing
riparian vegetation, but also interacted with woody debris
(vegetation influence) to create numerous LWD jams. Those
LWD accumulations, in turn, mediated the geomorphic
mechanisms of flood energy, water table, and soil chemistry by
providing seedling germination sites that were protected from
floods and had high moisture and nutrient availability.
Certain feedback scenarios may be constrained to specific
biogeographic locations. In Washington’s Olympic Mountains, Latterell et al. (2006) described a dynamic patch mosaic
of fluvial landforms, including a variety of channels, bars,
floodplains, and terraces, each with a characteristic vegetation
assemblage. The distribution of these landform/patches was
controlled by lateral channel migration, but channel behavior
is influenced by LWD accumulations. Woody debris accumulations in turn require key-member logs, contributed by erosion of mature terraces (classified based on stand age and
composition), along with smaller woody debris from other
landforms. They cautioned, however, that their model of
patch structure and dynamics would only be applicable in
comparable environments, and should not be applied to
dissimilar settings, such as those where vegetation distribution
is constrained by water availability or where trees do not reach
sufficient size for woody debris to play an important a role. In
the Transverse Ranges of California, Bendix and Cowell (2010)
found that the distribution of flood depth across valley floors
when integrated with species composition allowed for the
determination of the distribution and rate at which burned
snags fell after wildfire. This, in turn, governs the supply and
distribution of woody debris, with its attendant geomorphic
and ecological roles. The exogenous role of fire as a catalyst for
this set of feedbacks limits the applicability of these findings to
environments where riparian forests do in fact burn (Dwire
and Kauffman, 2003).
Attempts to quantitatively model riparian biogeomorphic
feedbacks are relatively new and typically require complex,
often recursive model structures. An analog is provided by
efforts in the field of hillslope modeling, in which representations of vegetation growth are coupled with surface runoff
and landscape evolution models (e.g., channel–hillslope integrated landscape development (CHILD) model, Tucker and
Bras, 1999; Tucker et al., 2001; Istanbulluoglu and Bras,
2005). An early numerical modeling effort for river channels,
about which much less work has been conducted than for
hillslopes, consists of work by Murray and Paola (1994,
1997), which explored channel braiding through simple numerical feedbacks between bedload transport and channels
with noncohesive banks. Over a series of modeling exercises,
they found that braiding occurred under conditions of sparse
or slowly growing vegetation, and high sediment fluxes were
caused by high discharge and/or steep regional slopes. In their
studies, the channels reorganized on time frames shorter than
the riparian plants’ life spans.
An alternative feedback quantification approach is
provided by Perucca et al. (2007), who coupled a fluid dynamic model of meandering rivers with a process-based
model of riparian biomass dynamics. In this approach, riparian vegetation influences channel morphology via a relationship between biomass, density, and bank erodibility. Their
numerical results show a strong effect of vegetation growth on
meander evolution and, like Murray and Paola (1994, 1997),
they emphasized the sensitivity of their results to the relative
temporal scales of plant growth versus morphodynamic
processes.
12.5.7
Patterns in Published Literature
The forgoing review illustrates that a large volume of research
has brought increasingly complex perspectives on ecogeomorphic interactions to the study of fluvial/riparian environments. That large volume suggests that a systematic analysis of
published work might reveal underlying patterns in what has
been learned to date. Accordingly, we undertook an analysis of
the past 20 years of published literature relating riparian
vegetation to hydrogeomorphic processes and/or specific fluvial landforms. This content analysis was designed to quantify
the incidence of published content according to a scheme that
was systematic and objective. Our particular interest was to
discern whether certain relationships tend to be revealed in
particular environments, or when studied at particular spatial
scales. Parker and Bendix (1996) called for research examining
the extent to which vegetation–landform relationships vary
geographically, suggesting that some might be generalized for
a variety of environments while others might be unique to
certain regions. This analysis of the literature offers one approach to that challenge.
There is a danger in such a study of detecting the patterns
of researchers’ interests, rather than actual variation in natural
processes. Nonetheless, the studies being published presumably have some relationship to the predominant processes
in the locales where they are conducted, so that patterns in the
types of interaction in the literature should offer some clues as
to the relative import of those interactions in different locations and at different scales.
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12.5.7.1
The Sample and Coding
As an initial step, we searched the Scopus and Web of Science
databases for all publications from 1990 through 2009 with
the term ‘riparian vegetation’ in the title, abstract, or keywords.
This search yielded 43700 references. We examined the abstracts of these papers and retained those that met the following criteria: (1) the paper included substantive analysis of
hydrogeomorphic interaction with the riparian vegetation; (2)
the study was data based, rather than a review or solely theoretical discussion; and (3) the data were collected in the field,
rather than laboratory, because the latter would not be inherently reflective of geographic or scalar variation.
For each of the papers that passed this filter, we recorded
the year of publication, the journal (or conference proceeding), the continent and country on which the data were collected, the study region’s ecological biome, the scale of
observation of the study, the causal direction of the environmental relationships studied, and the causal mechanism
found (for hydrogeomorphic influence only; see below). The
first four of these are rather straightforward, although country
was occasionally complicated by cross-boundary studies. For
biome, we classified each study as being in one of the 14
biomes identified by the Millennium Ecosystem Assessment
(2005; Figure 2).
Biomes
Tropical and subtropical moist broadleaf forest
Tropical and subtropical dry broadleaf forest
Tropical and subtropical coniferous forest
Temperate broadleaf and mixed forest
Temperate coniferous forest
Boreal forest / taiga
Tundra
Mediterranean forest, woodland, and scrub
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Coding of biome for each study was based on the description provided in the paper’s text. If such a description was
lacking, we located the study site on Figure 1 to determine the
appropriate biome. Scale was coded as ‘site’ if the data had been
collected at a single cross section involving o500 m length of
channel, as ‘reach’ if data were collected from a greater length of
the river, up to 1 km, and ‘watershed’ if the data were
from multiple sites or from 41 km length of the river.
Causal direction reflected the emphasis of the study:
if it focused on hydrological and geomorphic influences on
vegetation, it was coded as ‘geomorphic’; if it dealt primarily
with the impact of vegetation on geomorphology and/or hydrology, it was coded ‘vegetation’; and if the primary emphasis
was on feedbacks between the two, it was recorded as ‘feedback’.
Because so much research has been directed at disentangling the specific mechanisms of hydrogeomorphic influence on riparian vegetation, we further coded those studies
for which the causal direction was geomorphic, focusing on
the six categories of causal mechanisms described above
(Table 1). In each instance, we coded for the primary influence
identified in the paper. If the study did not find that a single
influence predominated, but rather that multiple factors were
exerting comparable degrees of influence, we coded it as
‘multiple’. We tried to be parsimonious in the use of this
classification, and although many papers mentioned potential
Tropical and subtropical grassland, savanna, and shrubland
Temperate grassland, savanna, and shrubland
Montane grassland and shrubland
Flooded grassland and savanna
Mangrove
Desert and xeric shrubland
Rock and ice
Figure 2 Biome boundaries used in the study. Based on the Millennium Ecosystem Assessment, 2005. Ecosystems and Human Well-Being:
Biodiversity Synthesis. World Resources Institute, Washington, DC, 86 pp.
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journals with multiple papers accounted for almost half
(136) of the total (Table 2). In general, the number of papers
on the topic has increased (Figure 3), albeit with some fluctuation. Over the past 5 years, more than 20 papers per year
have dealt with riparian vegetation–landform relationships.
The majority of the papers (189) examined geomorphic influences, with 62 being coded as vegetation, and just 23
covariables in passing, we did not code a study as multiple if
its focus seemed clearly to be on one mechanism.
12.5.7.2
General Characteristics of the Sampled Literature
There were 274 studies that met our criteria for inclusion in
the sample. The papers appeared in 101 venues, but 12
Table 1 Causal mechanisms coded for studies identified with the primary causal direction of geomorphic influence over vegetation
Causal mechanism
code
Criteria for classification
Energy
Sedimentation
Inundation
Water table
Hydraulic energy of floodwaters, whether destroying vegetation or eroding its substrate (Section 12.5.4.1)
Sediment deposition, whether by its impact on existing vegetation or by creation of new substrate (Section 12.5.4.2)
Extended inundation by floodwaters (Section 12.5.4.3)
Depth to the water table under the landform(s) on which the vegetation was growing, and variation in water-table depth.
(Section 12.5.4.4)
Soil chemistry of the landform(s) on which the vegetation was growing (Section 12.5.4.5)
Transport of plant propagules by floodwaters (Section 12.5.4.6)
Multiple factors were exerting comparable degrees of influence
Soil chemistry
Dispersal
Multiple
Table 2 Journals in which five or more of the sampled papers were published, by number and percent of the total sample, with number within
each causal direction
Journal
Number (%) of papers
Geomorphic
Vegetation
Feedback
River Research and Applications (and Regulated rivers)
Geomorphology
Earth Surface Processes and Landforms
Wetlands
Ecological Applications
Plant Ecology
Forest Ecology and Management
Journal of the American Water Resources Association
Journal of Vegetation Science
Physical Geography
Annals of the Association of American Geographers
Hydrological Processes
31 (11.3)
19 (6.9)
17 (6.2)
14 (5.2)
10 (3.6)
8 (2.9)
7 (2.6)
7 (2.6)
7 (2.6)
6 (2.2)
5 (1.8)
5 (1.8)
28
3
5
14
8
8
7
2
7
4
5
1
3
10
10
0
1
0
0
3
0
1
0
3
2
6
2
0
1
0
0
2
0
1
0
1
35
Number of studies
30
Geomorphic driver
Vegetation driver
Feedbacks
25
20
15
10
Year
Figure 3 Year of publication of the studies included in the sample, and the causal direction examined.
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
0
1990
5
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160
Xeric shrub
Temperate broad forest
Temperate conif forest
Mediterranean
Temperate grassland
Tropical grassland
Boreal forest
Tropical moist forest
Tropical dry forest
140
Number of studies
120
100
80
60
40
20
0
Africa
Asia
Australia
Europe
North
America
South
America
Figure 4 Number of studies conducted in each biome, on each continent.
feedback. Indeed, the interest in vegetation as driver is relatively recent, with few papers appearing before 1997. The
disparity in the causal direction studied is reflected in Table 2,
as only four of the journals with five or more papers in the
data set had a majority of their papers on vegetation’s influence: Geomorphology, Earth Surface Processes and Landforms,
Journal of the American Water Resources Association, and Hydrological Processes. It is logical that these journals, with their
emphasis on landforms and hydrology, tended to publish
studies in which the influences on those topics are examined.
However, the extent to which the rest of the literature emphasizes vegetation as the dependent variable is rather striking. The smaller number of studies addressing feedbacks may
well have a logistical explanation. Although most scientists
working in riparian environments would probably acknowledge the importance of feedbacks, to actually study them in
the field poses the challenge of incorporating two field studies
(vegetation driver and geomorphic driver) into one, in order
to discern their interactions. In addition, feedbacks take time
to observe in the field, as there must be time for an initial
process to occur, and then for a subsequent process to occur in
response.
There was also a distinct geographic bias to this literature.
The great majority of the studies was carried out in North
America, with a secondary peak in Europe (Figure 4). Australia, Asia, and Africa were the setting for substantially fewer
studies, with South America accounting for the least of all.
This pattern presumably reflects the fact that the majority of
the scholars publishing the studies are based at institutions in
North America, Europe and, to a lesser extent, Australia. In
turn, with more than 80% of the studies being conducted in
Europe (primarily Western Europe), North America, and
Australia, it is unsurprising that there is also an imbalance in
the biomes studied. Most study areas were located in temperate broadleaf and mixed forest (fairly widely distributed),
desert and xeric shrubland (mostly in North America), temperate coniferous forest (mostly in North America), and
Mediterranean forest, woodland and scrub (mostly in Europe
and western North America). The relative scarcity of papers in
the data set from South America, Africa, and Asia contributes to the underrepresentation of tropical biomes in our
survey.
Across all of the biomes, the studies tended toward larger
scales of observation, with watershed scale being the most
common and site scale being the least common (Figure 5).
Overall, flood energy was the most common mechanism
(28%), followed by water table and multiple (both 21%), and
sedimentation (12%). These were also the mechanisms that
were found to operate in most biomes. There were, however,
distinct differences in the spatial distribution of these mechanisms, as discussed below.
12.5.7.3
Differences among Biomes
The research within most of the biomes reflects the general
tendency of the literature to focus on the geomorphic causal
direction (Table 3). The two tropical and subtropical forest
biomes are exceptions to this generalization, although with
just 10 studies between them, they can hardly be said to
constitute a regional research trend. Our discussion, then,
deals primarily with the hydrogeomorphic influences on
vegetation.
The number of studies within each biome that focused on
each mechanism of hydrogeomorphic influence is shown in
Figure 6. A striking, and intuitive, feature of this graph is the
prominence of depth to water table in desert and xeric shrubland (44% of the studies in the biome). Where water is the
overriding limiting factor for vegetation, the proximity of
subsurface water becomes all the more important. The flashy
hydrology that characterizes many desert environments presumably accounts for the prominence of flood energy as a
mechanism in this biome, as high-energy floods are to be expected. Conversely, flood duration in most deserts tends to be
short, so that the 11% coded as inundation are more puzzling.
Some authors did simply discuss inundation as an important
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
Table 3 Number of papers by study area biome and causal direction
Biome
Number of papers
Geomorphic
Vegetation
Feedback
Temperate broadleaf and mixed forest
Desert and xeric shrubland
Temperate coniferous forest
Mediterranean forest, woodland, and scrub
Tropical and sub-tropical grassland, savanna, and shrubland
Temperate grassland, savanna, and shrubland
Tropical moist forest
Boreal forest
Tropical and sub-tropical dry broadleaf forest
77
58
49
38
21
14
8
7
2
44
45
38
28
15
11
1
6
1
28
6
9
5
4
2
7
1
0
5
7
2
5
2
1
0
0
1
variable without specifying whether the impact of the inundation was through the submergence or through mechanical
damage sustained during inundation, and this vagueness may
account for the high numbers of inundation references.
Mediterranean forest, woodland, and scrub have much in
common with desert and xeric shrubland, with the pronounced dry season making water a critical limiting resource,
and characteristically with similarly flashy hydrologic regimes.
In Mediterranean environments, flood energy predominated
as in xeric regions, but water table was much less prominent.
Many of the studies coded as multiple in this biome did include water table as one of the variables. Nonetheless, given
the prominence of obligate riparian species in Mediterranean
biomes, presumably present only due to the presence
of a (relatively) shallow water table, it is surprising that this
variable did not emerge in more studies. Sedimentation
was important here, reflecting the high sediment load and
sediment mobility typical of many Mediterranean streams.
Sedimentation may also be important because of the impact
of sediment texture on capillarity, which mediates soil moisture availability. Both the mobility of sediment and its importance for water availability are characteristic of streams in
most dry environments, so that the less frequent role reported
for sedimentation in desert studies is rather anomalous.
The temperate and the tropical/subtropical grassland, savanna, and shrubland had much in common. Each had flood
energy as the most prominent mechanism, with water table
and sedimentation also being important. Water table was,
however, distinctly more important in the tropical and subtropical grasslands than those in temperate zones. This likely
reflects the greater heat stress and consequent water demand
in the lower-latitude biome than in temperate areas. As in
other dry environments, high sediment mobility and the capillary influence of substrate texture are reflected in the importance of sedimentation.
The temperate broadleaf and mixed forest has the most
heterogeneous mix of mechanisms examined. This reflects two
factors acting in combination. One is simply that with so
many studies having been conducted in the biome, there is an
increased likelihood that most conceivable mechanisms will
have been examined. It is a type of environment in which
there is no overriding stressor (such as drought), allowing for
a range of individual mechanisms to prevail, depending on
the specific conditions in a given study area. Much the same is
true of the temperate coniferous forest. In each biome, flood
energy is the most notable mechanism, but is joined by water
table, sedimentation, dispersal, and inundation. The temperate broadleaf forest and the temperate grassland are the only
biomes in which soil chemistry appears as an influence.
The paucity of studies from tropical forests makes it difficult to infer much about these biomes. It is perhaps unsurprising that water-table depth was important to riparian
vegetation in tropical and subtropical dry broadleaf forest,
but with only one study conducted there is not enough to
establish a pattern. Again, with only one study from a loworder mountain stream in tropical and subtropical moist
broadleaf forest, citing multiple mechanisms, it is similarly
not informative.
Finally, the boreal forest/taiga, although also indicating a
role for flood energy, is notable for the prominence of dispersal. In this case, the pattern is clearly a result of the sample
characteristics, rather than an anomalously important role for
hydrochory in boreal forests. The sample for this biome was
small (Figure 6), and happened to be dominated by a group
of scholars in Sweden who published several papers on dispersal during the time period covered by our study (e.g.,
Andersson et al., 2000). Indeed, this example offers a useful
reminder that the interests of individual researchers have a
distinct potential to skew results for any biome (or scale) in
which the number of papers is limited.
12.5.7.4
Scale-Related Differences
The contrasts in mechanisms predominating in studies with
different scales of observation (Figure 7) are less dramatic
than among the varied biomes, but some differences are apparent. In keeping with their overall importance, flood energy
and water table are important across all scales. But both are
more prominent in studies at the site scale (o–0.5 km of
stream bank length) than at reach (0.5–1 km) or watershed
(41 km) scales. This is logical, as both have greater potential
variance at a given cross-section than along a gradient up or
downstream. Along a cross-section, they will vary from a
presumed maximum energy and minimum distance to water
table at or near the thalweg to zero energy and maximal watertable depth at some distance from the channel. This may also
be why inundation is so much more prominent at the site
scale. It is similarly logical that dispersal is irrelevant at the
individual site scale, where there is no space for transport to
occur, but increases in prominence with increasing scale. It is
less clear why no studies at the site scale focused on multiple
mechanisms.
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
67
No. of
studies
20
18
16
14
12
10
8
6
4
2
Multiple
Flood energy
0
Xe
s
ric
hr
ub
t
es
for
Water table
Sedimentation
t
es
for
d
an
oa
d
br
ne
nif
a
o
e
r
lan
t
c
r
nd
ra
ss
e
te
i
t
e
a
la
d
a
r
p
e
g
st
er
ss
m
M
e
p
a
re
e
t
T
m
fo
gr
ra
st
l
l
e
Te
a
re
p
st
ica
re
fo
m
p
o
e
re
o
T
B
ry
fo
d
Tr
t
l
s
oi
ica
lm
op
r
a
T
ic
op
Tr
Inundation
Dispersal
Chemistry
Figure 6 Mechanisms of hydrogeomorphic influence in studies within each biome.
60
Watershed
Reach
Site
Number of studies
50
40
30
20
10
le
tip
ul
M
D
is
pe
rs
al
n
tio
en
ta
m
Se
di
ab
le
W
at
er
t
at
io
n
In
un
d
en
oo
d
Fl
So
i
lc
he
m
is
try
er
gy
0
Figure 7 Number of studies emphasizing each mechanism at each scale.
12.5.7.5
Hydrogeomorphic Mechanisms in the Context of
Scale and Biogeography
The broad pattern of the mechanisms acting at varied scales in
different biomes can be seen in a plot of the modal mechanism
for each combination of scale and biome (Figure 8). This plot
serves to reinforce the primacy of flood energy as the most
common hydrogeomorphic driver affecting riparian vegetation.
However, it also reveals exceptions that indicate the importance
of both the setting and the scale of observation in determining
which drivers will predominate. In desert and xeric shrub, although flood energy is important (Figure 6), the overriding
importance of distance to the water table is present across all
scales.
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
Temperate conif forest
Flood
energy
Flood
energy
Flood
energy
Temperate broad forest
Water
table
Flood
energy
Flood
energy
Temperate grassland
Flood
energy
Flood
energy and
sedimentation
Flood
energy and
sedimentation
Sedimentation
Sedimentation
Flood
energy
Water table
Water table
Water
table
Flood energy,
inundation,
sedimentation
Water table
Flood
energy
Mediterranean
Xeric shrub
Tropical grassland
Site
Reach
Watershed
Figure 8 Modal mechanisms cited for each combination of scale and biome. Multiple mechanisms reflect scale–biome combinations for which
equal numbers of studies cited the mechanisms shown. Biomes in which fewer than 10 studies had been conducted are omitted from the figure.
In other biomes, the primary mechanism shifts with the
scale of the study. Flood energy appears most consistently at
the watershed scale. Clearly, the downstream changes in valley
morphology that can be found in many river systems across a
range of environments allow for sufficient variance in flood
energy for marked patterns of vegetation response to emerge
(Hupp, 1982; Bendix, 1997). At the site scale, however, water
table assumed the greatest importance for temperate broadleaf
and mixed forest. In this humid environment, a high water
table may prove to be a limiting factor, rather than a resource.
Thus, when viewed in detail across a given site, species with
specialized adaptations occur where the water table is highest
(e.g., Nakamura et al., 2002; Sharitz and Mitsch, 1993).
Sedimentation is the principal factor at the watershed scale
only for temperate grassland, but at decreasing scales appears
elsewhere, and is the predominant driver for the Mediterranean environment at both reach and site scale. The deposition
of sediment, whether burying plants (e.g., Wang et al., 1994)
or creating substrate (e.g., Greco et al., 2007), is in many instances a localized phenomenon, and its variance is likely to
be more evident at the site or reach scale than across a
watershed.
12.5.8
Patterns and Perceptions Revealed in the
Literature
Our review of the past 20 years of research shows that there has
been a steady and increasing volume of research on riparian
biogeomorphic relationships. Most of the studies in our sample
examined hydrogeomorphic influences on vegetation, despite
the longstanding realization that vegetation also affects fluvial
processes and landforms. This disparity is in part an artifact of
our sampling strategy: we included only field studies, and much
of the research on vegetation as a driver has been experimental,
typically using flumes. Furthermore, the use of riparian vegetation as our search term may have served to exclude some of
the studies examining the impact of plants on fluvial processes.
Whereas the modifier riparian is logical for a paper with an
ecological or biogeographic focus, it becomes redundant as a
description of vegetation in a paper on fluvial geomorphology,
for which all vegetation is likely to be riparian. The scarcity of
papers in the sample that address feedback is probably more
representative of the overall literature, as this is a topic that
received little attention until recently. The recent publication of
major theoretical articles specifically advocating recognition
and study of feedbacks (e.g., Stallins, 2006; Corenblit et al.,
2009) may well accelerate the increase in such work that was
noted by Murray et al. (2008).
The overall body of literature is dominated by empirical
studies detailing the impacts of flood energy and watertable distance on riparian vegetation. But both their relative
importance and the importance of other mechanisms do vary
with both the biome in which studies are set and the scale of
observation. This variance confirms Parker and Bendix’s
(1996) speculation that there is a geography of biogeomorphic interactions. Indeed, the nature of our sample may
well obscure that geography. Many of the world’s biomes
(Figure 2) are either underrepresented or entirely absent from
our sample.
The underrepresentation of key biomes is one of several
reasons for caution in interpreting our findings. Our emphasis
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
on identifying a single key factor in as many studies as possible inevitably required simplification of what were often
complex analyses. Once a key mechanism is identified from a
study, the question remains of whether that mechanism was
dominant in the environment (or at the scale) in which it was
studied, or was simply the mechanism most easily measured
or of most interest to the author(s).
Notwithstanding these caveats, we believe that there is
much to learn from this survey. Clearly, some spatial patterns
do exist in terms of spatial variation and the importance of
different types of biogeomorphic interaction. The empty
spaces in our data may be the most interesting of all: the
biomes that have not been studied, and the types of interactions that have not been studied within certain biomes,
point the way for future research.
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Biographical Sketch
Jacob Bendix is associate professor of geography in the Maxwell School at Syracuse University, and adjunct
associate professor of environmental forest biology at the State University of New York College of Environmental
Science and Forestry (SUNY-ESF). His research interests focus on fluvial geomorphology, disturbance ecology, and
riparian environments. He earned his BA from the University of California, Berkeley (geography), his MS from the
University of Wisconsin–Madison (geography, fluvial geomorphology emphasis), and his PhD from the University of Georgia (geography, biogeography emphasis).
Author's personal copy
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Riparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
John C Stella is assistant professor of forest and natural resources management at the State University of New York
College of Environmental Science and Forestry (SUNY-ESF), and holds an adjunct appointment (geography) in
the Maxwell School at Syracuse University. His research interests focus on riparian and stream ecology, dendroecology, plant ecohydrology and stable isotope biogeochemistry, and restoration ecology. He earned his BA
from Yale University (architecture), and his MS and PhD from the University of California, Berkeley (environmental science, policy and management). His research sites are located in semi-arid regions of California and the
US Southwest, Mediterranean Europe, and the Adirondack mountains of New York.